Field and Numerical Studies of Near-Bed Aggregate
Dynamics
W. Ritzrau and H. Fohrmann
1
Introduction
The small-scale distribution and the physical and biological modification of particulate matter influence near bottom transport processes of particles and consequently their incorporation in the sediment. Early on in sedimentology, major
focus was set on the size distribution of particles above the sea bed and in the
sediments. The vertical distribution of different particle size classes and its relationship to the hydrodynamic regime was empirically investigated (Rouse 1937).
Only recently have biologists become interested in the water mass above the sediment-water interface. However, the dynamics of particles, their interaction and
modification within the last few meters above the seafloor (the benthic boundary layer - BBL) is in many terms poorly understood.
Mc Cave (1984) proposed a more or less even distribution of the mass of the
various existing size classes of particles in the deep ocean and identified a hierarchy in the encounter processes controlling particle interaction close to the seafloor. Differential settling, the scavenging of small particles by larger faster-sinking particles (Stolzenbach 1993) is the dominant mechanism of particle interaction, followed by turbulent encounter and Brownian motion. However, the dominance of either of these processes is strongly dependent on the hydrodynamic
regime and the prevailing size distribution of particles.
The aggregation theory of particles was mainly applied to processes in the upper ocean, for example to explain sedimentation events of phytoplankton from
the euphotic zone (Jackson 1990). Hill and Nowell (1990) applied the mathematical formulations of particle encounter in the context of particles settling
through the water column. In well-controlled laboratory experiments they verified and improved the aggregation theory for different particles in the sub- and
super Kolmogorov size range. Only recently, they applied the aggregation theory
to the boundary layers of shelf environments (Hill and Nowell 1995). Following
the modification of a given particle community over time, they showed in numerical experiments that various processes have to be taken into account. The
general size distribution found in the ocean is not necessarily altered by particle
interaction. One basic assumption of their layer-averaged model is the absence
of strong vertical concentration gradients of particles in the simulated layer, an
Dynamics
W. Ritzrau and H. Fohrmann
1
Introduction
The small-scale distribution and the physical and biological modification of particulate matter influence near bottom transport processes of particles and consequently their incorporation in the sediment. Early on in sedimentology, major
focus was set on the size distribution of particles above the sea bed and in the
sediments. The vertical distribution of different particle size classes and its relationship to the hydrodynamic regime was empirically investigated (Rouse 1937).
Only recently have biologists become interested in the water mass above the sediment-water interface. However, the dynamics of particles, their interaction and
modification within the last few meters above the seafloor (the benthic boundary layer - BBL) is in many terms poorly understood.
Mc Cave (1984) proposed a more or less even distribution of the mass of the
various existing size classes of particles in the deep ocean and identified a hierarchy in the encounter processes controlling particle interaction close to the seafloor. Differential settling, the scavenging of small particles by larger faster-sinking particles (Stolzenbach 1993) is the dominant mechanism of particle interaction, followed by turbulent encounter and Brownian motion. However, the dominance of either of these processes is strongly dependent on the hydrodynamic
regime and the prevailing size distribution of particles.
The aggregation theory of particles was mainly applied to processes in the upper ocean, for example to explain sedimentation events of phytoplankton from
the euphotic zone (Jackson 1990). Hill and Nowell (1990) applied the mathematical formulations of particle encounter in the context of particles settling
through the water column. In well-controlled laboratory experiments they verified and improved the aggregation theory for different particles in the sub- and
super Kolmogorov size range. Only recently, they applied the aggregation theory
to the boundary layers of shelf environments (Hill and Nowell 1995). Following
the modification of a given particle community over time, they showed in numerical experiments that various processes have to be taken into account. The
general size distribution found in the ocean is not necessarily altered by particle
interaction. One basic assumption of their layer-averaged model is the absence
of strong vertical concentration gradients of particles in the simulated layer, an